1. Field of the Invention
The present invention relates to DC offset compensation in a frequency mixer device that mixes an input signal and a switching signal, and outputs a multiplied signal. It is particularly related to a frequency mixer device having a DC offset compensating function suitable for compensating DC offset generated when an interfering waveband signal is input when using a direct conversion wireless receiver, and a method for compensating the DC offset.
2. Description of Related Art
In recent years, technology that uses a direct-conversion system has been proposed in connection with the miniaturization and price reduction of wireless receivers. In this system an input RF (wireless frequency) signal is converted directly to a low frequency baseband signal, and so in comparison to a super heterodyne system that requires a high frequency IF (intermediate frequency) signal, it has the advantage that an intermediate frequency filter becomes unnecessary. A direct conversion system is also referred to as a zero IF system, because the center frequency of the IF signal is DC.
Frequency conversion is performed by mixing (compositing) the frequency of an input RF signal with a local signal having the same frequency using a mixer circuit. However, in a direct conversion system, when the input signal level is large, DC offset occurs in the output baseband signal when second order nonlinear distortion is present in the mixer circuit. This condition will be explained in detail with reference to FIG. 7 and FIG. 8.
FIG. 7 shows the spectrum of an input RF signal, with numeral 101 denoting a weak-level desired waveband signal with the center frequency being the same as a local signal frequency fLO, and numeral 102 indicating a high-level interfering waveband signal that is present at a higher frequency fINT. As a result of inputting an RF signal that accompanies this sort of high-level interfering waveband signal to a mixer circuit, the spectrum of the output signal appearing in the mixer output becomes as shown in FIG. 8. Numerals 103 and 104 respectively indicate the components that appear in the mixer output after frequency conversion of the desired waveband signal 101 and the interfering waveband signal 102 of the RF input. Numeral 105 indicates the DC offset generated by the high-level interfering waveband signal when second order nonlinear distortion is present in the mixer circuit.
Accordingly, in a direct conversion system, there is the problem that receiver sensitivity decreases due to the DC offset 105 generated in the frequency range of the desired waveband signal 103 of the mixer output. If the mixer circuit is composed of a differential circuit and the differential balance is completely symmetrical, second order nonlinear distortion will not be present. However, because the components constituting the differential circuit cannot be made completely symmetrical due to manufacturing irregularities, it is not possible to eliminate second order nonlinear distortion. Therefore, technology has been proposed that compensates the DC offset generated by second order nonlinear distortion.
A method for detecting an interfering waveband signal included in the input RF signal and compensating DC offset generated in the mixer output, disclosed in U.S. Pat. No. 6,535,725, is explained below with reference to FIG. 9.
In FIG. 9, numeral 106 indicates a mixer circuit, which is composed of a switching cell 107 and an RF input cell 108. The switching cell 107 is composed of bipolar transistors Q1, Q2, Q3, and Q4. The RF input cell 108 is composed of bipolar transistors Q5 and Q6, and resistors R. The RF signal input from RF input terminals 109 and 110 is amplified by the RF input cell 108. The amplified RF signal is converted to an IF signal by being mixed with a local signal input from local input terminals 111 and 112 in the switching cell 107, and this converted IF signal is output from output terminals 113 and 114.
If all of the transistors Q1, Q2, Q3, and Q4 constituting the switching cell 107 have exactly the same characteristics, balance as a differential circuit will be completely symmetrical. However, because the bipolar transistors Q1, Q2, Q3, and Q4 each individually have properties that differ from the ideal properties due to manufacturing irregularities, second order nonlinear distortion is generated when the input RF signal is converted to an IF signal. Therefore, DC offset is generated in the mixer output as shown in FIG. 8. As is well known, because the DC offset is proportional to the square of the input signal strength, the higher the level of the interfering waveband signal included in the input signal, the greater the output DC offset will become.
On the other hand, the circuit shown in FIG. 9 is provided with a DC offset compensator 115. The DC offset compensator 115 is composed of a detector 116, a controller 117, and a correction generator 118. The detector 116 detects an input RF signal and outputs a detection signal. The controller 117 generates a control signal in response to the detection signal. The correction generator 118 generates a compensation signal in response to the control signal from the controller 117 such that it reduces the DC offset of the mixer output terminals 113 and 114. By this operation of the DC offset compensator 115, the compensation signal that the correction generator 118 outputs to the mixer circuit 106 changes in response to the strength of the RF signal input, and the DC offset of the mixer output is cancelled. Further, in the mixer circuit 106, because the second order nonlinear distortion properties of each individual element differ due to manufacturing irregularities, a function of a user interface 119 is also provided in the DC offset compensator 115 in order to adjust the control signal produced by the controller 117.
However, in the method that adds a compensation signal in order to compensate the DC offset included in the mixer output signal, a low frequency noise generated within the DC offset compensator is superimposed on the compensation signal that is used. Therefore, not only the DC offset included in the mixer output signal being compensated, but also a noise signal is newly added in the frequency range of the desired waveband signal. In order to explain this condition, the spectrum of the mixer output after DC offset compensation is shown in FIG. 10. Numerals 103 and 104 respectively indicate the desired waveband signal and the interfering waveband signal appearing in the mixer output after frequency conversion in the mixer circuit, as shown in FIG. 8. A numeral 120 indicates the low frequency noise included in the compensation signal generated by the DC offset compensator, which ranges over the desired waveband. Therefore, the degradation of receiving sensitivity when inputting a high-level interfering waveband signal is not improved even when DC offset is compensated.